The invention relates to a display device comprising a light guide, a movable element and selection means to locally bring said movable element into contact with the light guide.
A display device of the type mentioned in the opening paragraph is known from United States patent application U.S. Pat. No. 4,113,360.
In said application a description is given of a display device comprising a first plate of a fluorescent material, in which, in operation, light is generated and trapped (so that this plate forms a light guide), a second plate which is situated at some distance from the first plate and, between said two plates, a movable element in the form of a membrane. By applying voltages to addressable electrodes on the first and second plates, the movable element can be locally brought into contact with the first plate, or the contact can be interrupted. A transparent contact liquid is present on the contact surfaces. At locations where the movable element is in contact with the first plate, light is decoupled from said first plate. This enables an image to be represented. If the movable element is not in contact with the light guide, it is in contact with the second plate.
For the proper functioning of the display device, it is important that the contact between the light guide and the movable element can be brought about and interrupted in an accurate manner. If it is impossible to interrupt a contact, then a pixel always or never emits light (dependent upon which contact cannot be interrupted). It will be obvious that this has a substantial adverse effect on the picture quality.
It is an object of the invention to provide a display device of the type mentioned in the opening paragraph, which exhibits an improved picture quality.
To achieve this, the display device in accordance with an embodiment of the invention is characterized in that said display device comprises means for reducing adhesive forces between the movable element and the light guide.
A problem which is encountered is that the force with which the movable element is brought into contact with the light guide and/or the second plate may be subject to variations, for example, because the movable element adheres (sticks) to the light guide or the second plate. As a result, the forces necessary to bring about or interrupt the contact between the light guide and the movable element cannot be predetermined. Even if all contacts can be interrupted, variations in the necessary forces cause a reduction of the picture quality, because the moment when a contact is interrupted is co-determined by the force necessary to interrupt the contact and hence variations in the necessary forces result in variations in the time period during which a pixel emits, or does not emit, light.
Such variations adversely affect the quality of the image displayed.
A reduction of the adhesive forces between the movable element and the light guide causes the forces necessary to bring about or interrupt the contact between the movable element and the light guide to become smaller, less subject to variations and better controllable. By virtue thereof, the picture quality can be improved. Within the scope of the invention, the term adhesive forces is to be taken to mean forces which are not brought about by the (settings of) the selection means.
An embodiment of the display device in accordance with the invention is characterized in that the movable element is situated in an evacuated space. An evacuated space is to be taken to mean, in this application, a space with a pressure below {fraction (1/10)} atmosphere, preferably below 10 Torr.
This embodiment of the invention is based on the insight that between the lower side of the movable element and the light guide an under-pressure may develop relative to the (atmospheric) pressure at the upper side of the movable element. The pressure difference between both sides of the movable element causes a force to be exerted on the movable element, which force presses said movable element against the light guide. The size of this force is governed by the pressure difference and varies according to the atmospheric conditions and the contact between the movable element and the light guide and, hence, is uncontrollable. Since, in this embodiment, the light guide and the movable element are situated in an evacuated space, the possible pressure difference and hence said force are reduced. In this embodiment, the means for reducing the adhesive forces is constituted by the evacuated space.
An embodiment of the display device in accordance with the invention is characterized in that the movable element comprises means for removing a static charge from the movable element.
This embodiment of the invention is based on the insight that a static charge may locally form on the movable element. As a result of this charge, the movable element is locally or entirely subject to an electrostatic force which is uncontrollable. By precluding the formation of a static charge, this force is reduced or eliminated.
Preferably, the movable element is electrically conducting and at a fixed potential.
An embodiment of the display device in accordance with the invention is characterized in that the movable element is provided with an anti-adhesion layer on the side facing the light guide and/or the light guide is provided with an anti-adhesion layer on the side facing the movable element.
This embodiment of the invention is based on the insight that chemical adhesive forces may develop locally between the movable element and the light guide. As a result of these forces, the movable element is locally or entirely subject to a force which is uncontrollable. By using an anti-adhesion layer on the movable element and/or the light guide, these forces are reduced or eliminated. The anti-adhesion layer may be, for example, a Teflon layer or a parylene (poly-p-xylylene) layer.
An embodiment of the display device in accordance with the invention is characterized in that the surface of the movable element facing the light guide and/or the surface of the light guide facing the movable element have/has an average roughness (Ra) above 5 nm, and neither of the two surfaces has a roughness in excess of 0.1 micrometer. Preferably, the roughness of both said surfaces ranges from 20 to 100 nm. If the roughness of both surfaces is below 5 nm, then van der Waals"" forces occur between said surfaces. Such van der Waals"" forces are large. If at least one of both surfaces exhibits a roughness above 5 nm, then van der Waals"" forces are reduced substantially or are absent. If one of both surfaces exhibits a roughness above 0.1 micrometer, then the optical contact between the movable element and the light guide is reduced.
Preferably, there is no liquid between the movable element and the light guide. Although a liquid improves the optical contact between both parts, it also brings about an adhesive force (as a result of surface stresses) between the movable element and the light guide. This force depends on the manner in which the movable element and the light guide are in contact with each other and on properties of the liquid as well as on other conditions such as pressure and temperature. Consequently, the above-mentioned adhesive force is subject to variations.
In an embodiment, the selection means comprise transparent electrodes, and the movable element, if it is in contact with the light guide, contacts said light guide at the location of an electrode, causing light to emit through the transparent electrode.
In this embodiment, the movable element and the light guide are in contact with each other at the location of an electrode. This embodiment has the advantage, compared to display devices in which contact takes place next to an electrode, that the force with which the contact is brought about at the location where light emission takes place is larger and better controllable.
Preferably, for each of the above embodiments, the movable element is arranged between the light guide and a further element, and the selection means comprise means for locally generating a force causing the movable element to move towards the light guide as well as a force causing the movable element to move towards the further element. This has the advantage, compared to embodiments in which the repellent force or forces (the force with which the movable element is pushed away from the light guide) is/are formed by elastic forces in the movable element, that elastic forces depend upon the elastic properties of the movable element, and these elastic properties may exhibit variations as a result of, for example, differences in thickness, material, temperature, etc.
A display device in accordance with the invention may comprise aspects of one or, preferably, more of the above embodiments.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.